Near-threshold $\eta$ production in $pp$ collisions

We study near-threshold $\eta$ meson production in $pp$ collisions within an effective Lagrangian approach combined with the isobar model, by allowing for the various intermediate nucleon resonances due to the $\pi$, $\eta$, and $\rho$-meson exchanges. It is shown that the $\rho$-meson exchange is the dominant excitation mechanism for these resonances, and the contribution from the $N^*(1720)$ is dominant. The total cross section data can be reasonably reproduced, and the anisotropic angular distributions of the emitted $\eta$ meson are consistent with experimental measurements. Besides, the invariant mass spectra of $pp$ and $p\eta$ explain the data well at excess energy of 15 MeV, and are basically consistent with the data at excess energy of 40 MeV. However, our model calculations cannot reasonably account for the two-peak structure in the $p\eta$ distribution at excess energies of 57 and 72 MeV, which suggests that a more complicated mechanism is needed at higher energy region.